1. Field of the Invention
This invention relates to a method of and a system for controlling idling speed in an electronically controlled engine, and more particularly to improvements in controlling idling speed of an electronically controlled engine wherein idling speed is feedback controlled in accordance with the operating conditions of the engine. The invention is especially useful with an electronically controlled engine of a motor vehicle provided with an electronically controlled fuel injection system.
2. Description of the Prior Art
Recently, along with advancement in electronic control techniques, and particularly with the advent of digital control techniques, so-called electronically controlled engines have been put into practical widespread use. The air-fuel ratio of such an electronically controlled engine is controlled by means of an electronically controlled fuel injection device. In electronically controlled engines of this type, a fuel injection period is typically determined in accordance with engine intake air flow rate or engine intake pipe pressure and in accordance with engine speed. Fuel injector valves provided on the intake manifold of the engine inject fuel towards engine intake ports. These fuel injector valves are opened only for an electronically controlled fuel injection period, so that the air-fuel ratio of the engine may be accurately controlled. Such electronically controlled fuel injection devices have been widely used in engines for motor vehicles, in which air-fuel ratio must be precisely controlled to obtain acceptably low exhaust emissions.
In such an electronically controlled engine, air intake flow rate while the engine is idling (during which time the vehicle is generally stationary) is controlled by an electronic feedback control unit in accordance with a difference between the engine speed and a predetermined target idling speed. Thus, engine idling speed is controlled using feedback (closed-loop) control techniques. The electronic control unit controls air intake flow rate by controlling the degree to which an idling speed control valve ("ISCV") is open, this valve bypassing the throttle valve to provide air to the engine during idling and at other times.
The idling speed control described above makes it possible to accurately control the idling speed of the engine. However, when the vehicle speed detecting system (e.g. vehicle speed sensor) fails and no vehicle speed signals are inputted to the electronic control unit, the electronic control unit judges that the vehicle is stationary. Even though the vehicle may actually be moving, the electronic control unit causes the idling speed control valve ("ISCV") to close, thus causing the flow rate of air passing through the ISCV to decrease. This inappropriate control response may cause problems such as engine stalling during deceleration of the vehicle.
More specifically, under normal conditions (as shown graphically in FIG. 1), a feedback process for closing the ISCV to thereby cause engine speed to approach a predetermined target idling speed is effected only when the vehicle is stationary. When the vehicle is moving, the idling speed feedback control process is discontinued, so that satisfactory control of engine idling speed occurs even when the vehicle is stopped after traveling. In contrast, when the vehicle speed detecting system (the vehicle speed sensor, for example) fails, the electronic control unit judges that the vehicle is stopped even when the vehicle is moving. In consequence, when the vehicle begins to decelerate and the throttle valve is closed, the electronic control unit limits engine speed to an upper limit to prevent excessive rotations during a given period. The controller is "fooled" into determining that the engine is idling (from a time t.sub.1 shown in FIG. 2), resulting in the closure of the ISCV. Since the feedback process for matching the engine speed to the target idling speed is performed continuously, the flow rate of intake air passing through the ISCV is further decreased from a time t.sub.2 to a time t.sub.3. In consequence, while the velocity of the vehicle and the engine speed is constant or increasing, no particular problems occur since intake air may flow into the engine by passing through the throttle valve. However, when the engine throttle valve closes to bring when the vehicle to a stop or to slow the vehicle, the flow rate of the engine intake air becomes considerably less than that required to operate the engine (since no air is being supplied through the now-closed ISCV), and the engine may falter and stall at a time t.sub.4.
In order to obviate the above-described disadvantages of the prior art, it has been proposed to keep the engine speed above or equal to a preset lower limit when it is judged that the vehicle speed detecting system has failed. However, in some cases, such a proposal has proved to be ineffective depending upon the running condition.